I can't agree with you about not spaying...It isn't always easy nor feasible to keep your pet away from 'fertile' animals. Most people keep their dogs outdoors, where any roaming 'fertile' dog can come in contact with. And don't think just because they are in a fence that that will keep a 'fertile' dog in or out for that matter.
Cats can roam freely, fence or no...
I have worked with local rescue groups for a few years as foster homes for strays. There are too many animals that are put down needlessly each year because of unwanted pregnancie and over population. There are not enough responible educated people to adopt all these animals and give them good homes, so millions are dying. Whether at the local shelter, or out on the streets; starving, being hit by vehicles, from diseases...etc.
All of my furry friends have been spayed or neutered. They were all between 6 - 8 months old before this happened to have enough hormones for growth. They are all well adjusted, content, happy animals. Happy to be safe and alive living with me. I know that they are happy...they tell me so.